Conventional track structures for guiding and supporting rollers on sectional overhead doors such as utilized in garage door systems pose several problems, particularly when a relatively small thickness or cross sectional area of metal material is utilized in the track structure that includes a track and track support members. These problems include (1) the tendency of the track to "bow", i.e. deflect in a horizontal direction toward the middle of the door, when significant weight is applied to the track through the door rollers, (2) the tendency of the track to bend or deflect downward in a vertical plane over large unsupported spans, (3) the tendency of the track trough to deform near the roller including widening of the trough and possible crimping of the edges due to the heavy weight of the door, or due to roller misalignment, and (4) damage to the exposed blade edges of the track structure during manufacture, shipping/handling and installation. Each one of these problems is discussed in sequence below.
Track bowing is caused by the way the door roller interacts with the conventional track and angle support system. This problem can lead to a condition known throughout the industry as "roll-out". This is when the door roller literally rolls out of the track trough. This condition can cause failure of the door to open or close properly, or even worse, cause the door to fall out of the track.
The second problem is that too much downward deflection causes the supporting hardware on the rollers to drag on the track resulting in a door not opening or closing smoothly.
The third problem is specifically related to a deficiency in the conventional track trough geometry itself. The conventional track configuration typically has an outer blade edge on the track that tends to have weak points wherever any imperfections exists. These points become stress concentration points or focal points where failure may occur due to heavy loads applied through the rollers.
The fourth problem is directly related to the third problem in that even minor damage sustained during shipping and handling, especially to the blade edge of the track can easily cause weak points in the track edge as set forth above.
There have been several approaches within the industry to try and address the above problems. Virtually all approaches have included increasing track depth, thickness, or both. While these approaches are simple, they have resulted in substantially heavier and thus more costly track systems. In contrast, the present novel track system uses a substantially different set of principles to address the above problems, without resorting to the use of a heavier, and thus more costly system.
The novelty and uniqueness of the present invention is that it maximizes the use of material through configuration synergisms, i.e. features that interact and play multiple roles simultaneously, such as contributing substantially to the moment of inertia about the vertical and horizontal axes, while also greatly increasing the resistance of the configuration to local damage and stress concentrations. The result is a dramatic increase in overall performance and efficiency that overcomes the problems set forth. The synergisms are so significant that the combined system achieves unexpected levels of material savings.
In order to better understand the novelty and uniqueness of the present invention, and to more fully appreciate how conventional track made of thinner materials fails in addressing the above four problems, and more specifically problems three and four, it is important to understand failure initiation and propagation in a conventional track system. This is discussed in more detail as follows.
In a conventional track, failure may be generally associated with two fundamental regions of high stress. The first region is associated with failure initiation, and the second is associated with failure propagation. The first region is an inherently characteristic region of edge stress concentration at the "blade edge" of the trough nearest to the roller contact point. This edge stress concentration is characteristic of the overall cross-sectional geometry of the "trough" of the track in which the roller rides. The second region is located in an area between the point of roller contact and the blade edge of the trough. In most commonly found sectional overhead door track sizes, this region is approximately one inch wide. This region is characterized by two stress peaks separated by a short distance along the line of roller travel. In most commonly found overhead door track sizes and weights, these two points are separated by approximately three-fourths of an inch, with one peak located symmetrically on either side of the point of roller contact.
Even the most perfect, smooth trough edge of conventional track will experience a very localized point of high stress gradient due to the characteristic edge stress concentration. Initiation of an edge "bulge" or "crimp" on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or "propagate" easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local imperfections, even those on the order of size of the thickness of the track itself.
Thus, the existence of any edge imperfections in a conventional track have the effect of enhancing an already established process of failure initiation. These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the roller load level at which failure initiates. This is the established process.
In a conventional overhead door track, failure propagation follows failure initiation in the following manner. Once a local "bulge" initiates at the blade edge, in the direction away from the roller contact point, the existence of the second region of high stress enables crimping of the blade edge to propagate. The result is a triangular "tea pot spout" shape which is formed as the edge folds distinctly along two lines connecting the first region of high stress of the blade edge with each of the two peaks of the second region. This propagation can be described as a local "edge buckling" since it is an instability of the metal sheet at the edge.
It should be noted that the propagation process described here corresponds to the case of a roller that is not rolling, but stays in the same position on the track as the load is increased until failure is reached. Actual in-service failures which may involve moving rollers will display variations of this basic propagation mechanism.
Consequently there is a demand in the sectional overhead door industry including both garage doors and vehicle doors for a cost effective, retrofitable track system made of thinner material that simultaneously addresses the four problems stated above in addition to resisting the failure sequence noted, while yet maintaining a high degree of manufacturability.